NATO: Reinforcing its eastern flank
NATO has been increasing its military presence along its eastern borders since 2014 and in response to Russian action.
Russia’s annexation of Crimea in 2014 prompted NATO to initiate what former Secretary General Jens Stoltenberg described as the “most significant strengthening” of its collective defence in decades.
Up until this point, NATO was focused on leading the international coalition in Afghanistan.
Russia’s actions in Ukraine brought NATO’s attention squarely back to its core mission of the collective defence of its members and the security of the Euro-Atlantic area.
A continuous presence in the eastern part of the AllianceNATO responded to Russia’s actions in Ukraine in 2014 by establishing a continuous air, land and maritime presence in the eastern part of the Alliance – its eastern flank. Initially, this involved increasing the number of air policing and surveillance aircraft in NATO airspace, a more prominent naval presence in the Baltics and Black Sea, and more frequent and larger military exercises.
The land element came later, with the creation in 2017 of four multinational battlegroups of 1,000 troops apiece in Estonia, Latvia, Lithuania and Poland.
In early 2022, NATO countries began to increase their military forces in the eastern part of the Alliance, both under the NATO umbrella and on a bilateral basis.
A bigger Response ForceIn 2014 NATO also decided to adapt its military posture to strengthen the ability of the Alliance to respond to any crisis that may occur, and to any challenge by Russia.
NATO expanded the existing Response Force to 40,000 personnel, creating within it a new very high readiness force of around 5,000 troops. New commands and logistics units were also established to help facilitate the movement of troops and equipment across the Atlantic Ocean and European territory.
Response to the Russian invasion of UkraineIn early 2022, NATO decided to reinforce its existing presence in its eastern flank, including additional personnel, fighter aircraft and ships.
Following Russia’s invasion of Ukraine on 24 February, NATO subsequently activated elements of the Response Force for the first time in a deterrence and defence posture. NATO leaders also agreed to establish four additional new battlegroups in central, eastern, and south-eastern Europe, taking the total to eight.
Subsequent decisions will allow all eight battlegroups to be ‘scaled up’ to brigade sized units when and where required. Rapid reinforcement of the alliance’s eastern flank will also be achieved, if necessary, through a new NATO force model which was adopted in 2024 and replaces the NATO Response Force.
At the Washington summit in July 2024, NATO leaders said that while the threat posed by Russia will “persist into the long term” the alliance “does not seek confrontation and poses no threat to Russia”.
Reinforcing the eastern flank in 2025In September 2025, NATO airspace was violated on several occasions by Russian drones and fast jet aircraft.
The NATO Secretary General, Mark Rutte, has said that while the incidents in Poland and Estonia were the largest concentrations of violations of NATO airspace, none of them were isolated occurrences and that “Russia’s recklessness in the air along our eastern flank is increasing in frequency”. Russia has denied deliberately targeting NATO countries.
In response to the Russian incursions into Polish and Estonian airspace, the North Atlantic Council has been convened twice in as many weeks under Article 4 of the Washington Treaty, which provides that member states can request consultations when they feel their security is threatened
NATO has launched a new surveillance and air defence operation along its eastern flank (Operation Eastern Sentry). NATO’s Supreme Allied Command for Europe, General Alexus Grynkewich, has said that while the operation is initially expected to focus on Poland, he envisaged it covering the entire eastern flank of the alliance, including the High North. The accession of Finland to NATO in April 2023 more than doubled NATO’s land border with Russia and added a further 830 miles to the alliance’s eastern flank.
Several allies, including the UK, Germany, France and Denmark have announced the deployment of additional aircraft and capabilities in support of the operation.
About this paper and further readingThis paper focuses exclusively on the measures undertaken by NATO to increase its military presence along its eastern flank since Russia annexed Crimea in 2014. It does not discuss political decisions nor Russian activity during this time. Further papers related to Russia’s invasion of Ukraine, including a Ukraine timeline and the provision of military assistance to Ukraine, are collated on the Commons Library page: The Conflict in Ukraine.